Influence of Coadsorbed Hydrogen on Ethylene Adsorption and

The effect of surface-bound hydrogen adatoms on adsorption, desorption, and ... Site-blocking effects of preadsorbed H on Pt(111) probed by 1,3-butadi...
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Langmuir 2005, 21, 971-975

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Influence of Coadsorbed Hydrogen on Ethylene Adsorption and Reaction on a (x3×x3)R30°-Sn/Pt(111) Surface Alloy Haibo Zhao and Bruce E. Koel* Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482 Received August 3, 2004. In Final Form: October 6, 2004 The effect of surface-bound hydrogen adatoms on adsorption, desorption, and reaction of ethylene (CH2d CH2) on a (x3×x3)R30°-Sn/Pt(111) surface alloy with θSn ) 0.33 was investigated by using temperatureprogrammed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed H decreased the saturation coverage of chemisorbed ethylene and less H was required to completely block ethylene chemisorption on this alloy than that on Pt(111). This is also the first report of extensive H site-blocking of ethylene chemisorption on Pt(111). Preadsorbed H also decreased the desorption activation energy of ethylene on the alloy surface. The reaction chemistry of ethylene on this Sn/Pt(111) alloy is dramatically different than on the Pt(111) surface: the H-addition reaction channel taking ethylene to ethane on Pt(111) is totally inhibited on the alloy. This is important information for advancing understanding of the surface chemistry involved in hydrogenation and dehydrogenation catalysis.

1. Introduction Hydrogenation of ethylene (C2H4) to ethane (C2H6) over Pt(111) has been studied by a number of investigators as a model system for understanding the surface science of catalytic hydrogenation reactions.1-6 On Pt(111) at 52 K under UHV conditions, ethylene chemisorbs molecularly via nearly complete rehybridization from extensive backbonding to eliminate the C-C π-bond and form two Pt-C σ-bonds.2 This di-σ-bonded ethylene species is sp3-hybridized7 and resides in a fcc 3-fold hollow site on the Pt(111) surface, as determined by low-energy electron diffraction (LEED).8 Heating the surface causes di-σ-bonded ethylene to begin to dehydrogenate to form surface-bound ethylidyne (CCH3) species at 240 K9 via a pathway that includes isomerization to ethylidene (CHCH3) as first proposed by Koel and co-workers10,11 and later supported by others.9,12 Coadsorption of hydrogen adatoms and C2H4 on Pt(111) weakens the chemisorption bonding of ethylene to the surface and greatly increases the production of ethane in TPD experiments.3,13 Somorjai and co-workers14 have studied ethylene hydrogenation at 300-370 K with the total pressure of 110 Torr and proposed that the reaction mechanism of ethylene hydrogenation over Pt(111) does not involve ethylidyne, which is a spectator species or is * Author to whom correspondence should be addressed. Phone: 1-213-740-4126. E-mail: [email protected]. (1) Boronin, A. I.; Bukhtiyarov, V. I.; Kvon, R.; Chesnokov, V. V.; Buyanov, R. A. Surf. Sci. 1991, 258, 289. (2) Casuto, A.; Mane, M.; Jupille, J. Surf. Sci. 1990, 249, 8. (3) Godbey, D.; Zaera, F.; Yeates, R.; Somorjai, G. A. Surf. Sci. 1986, 167, 150. (4) Marinova, T. S.; Chakarov, D. V. Surf. Sci. 1987, 192, 275. (5) Somorjai, G. A. Z. Phys. Chem. (Munich) 1996, 197, 1. (6) Zaera, F.; Janssens, T. V. W.; Ofner, H. Surf. Sci. 1996, 368, 371. (7) Ibach, H.; Lehwald, S. Surf. Sci. 1982, 117, 685. (8) Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942. (9) Cremer, P. S.; Stanners, C.; Niemantsverdriet, J.; Shen, Y. R.; Somorjai, G. A. Surf. Sci. 1995, 328, 111. (10) Windham, R. G.; Bartram, M. E.; Koel, B. E. J. Phy. Chem. 1988, 92, 2862. (11) Carter, E. A.; Koel, B. E. Surf. Sci. 1990, 226, 339. (12) Deng, R.; Herceg, E.; Trenary, M. Surf. Sci. 2004, 560, L195. (13) Berlowitz, P.; Megiris, C.; Butt, J. B.; Kung, H. K. Langmuir 1985, 1, 206. (14) Zaera, F.; Somorjai, G. A. J. Am. Chem. Soc. 1984, 106, 2288.

involved in a decomposition pathway. They also reported that relative hydrogenation rates of species adsorbed on Pt(111) at 295 K during ethylene hydrogenation at 1 atm are in the order of π-bonded ethylene . di-σ-bonded ethylene . ethylidyne.8 Commercially, other metal or metals are added as promoters to supported Pt catalysts. Pt-Sn bimetallic catalysts using a neutral (chemically inactive) support have been reported to exhibit high selectivity and catalyst stability for the dehydrogenation of light paraffins at elevated temperatures.15-17 Model studies of ethylene hydrogenation on unsupported Pt-Sn bimetallic surfaces reported that the reaction rate, i.e., ethylene turnover frequency (TOF), at 15 Torr and 300 K reached a maximum at θSn ) 0.1. This corresponded to an activity enhancement of 75% over that of clean Pt(111).18 Verbeek and Sachtler19 reported years ago that ethylene chemisorption was weakened on polycrystalline Pt/Sn alloys compared to that on Pt. Later, surface science studies probed ethylene adsorption on well-defined, ordered Pt-Sn alloys.20,21 Alloying Sn and Pt weakens the interaction between ethylene and Pt at the surface, and ethylene is less strongly chemisorbed and less rehybridized from the gas phase on both a (2 × 2)Sn/Pt(111) surface alloy with θSn ) 0.25, with a composition corresponding to the (111) face of a bulk Pt3Sn crystal and a (x3×x3)R30°Sn/Pt(111) surface alloy with θSn ) 0.33 and a composition corresponding to a Pt2Sn surface. While the nature of the chemisorbed species can still be regarded as di-σ-bonded ethylene, no decomposition of ethylene occurs on either of these two alloys during heating in TPD.20 There are no reports of ethylene and hydrogen coadsorption on Pt/Sn alloys, but one might expect copious production of ethane on the two Sn/Pt(111) surface alloys in coadsorption experiments. This could arise from a (15) Miller, S. J. U.S. Patent 4,727,216, 1986. (16) Brinkmeyer, F. M.; Rohr, D. F. U.S. Patent 4,866,211, 1987. (17) Imai, T.; Hung, C. W. U.S. Patent 4,430,517, 1983. (18) Park, Y. K.; Ribeiro, F. H.; Somorjai, G. A. J. Catal. 1998, 178, 66. (19) Verbeek, H.; Sachtler, W. M. H. J. Catal. 1976, 42, 257. (20) Paffett, M. T.; Gebhard, S. C.; Windham, R. G.; Koel, B. E. Surf. Sci. 1989, 223, 449. (21) Tsai, T.; Xu, C.; Koel, B. E. Surf. Sci. 1997, 385, 37.

10.1021/la048035q CCC: $30.25 © 2005 American Chemical Society Published on Web 12/24/2004

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confluence of the large increase in ethane production observed in TPD after H + C2H4 coadsorption on Pt(111), the higher relative hydrogenation rate reported for π-bonded ethylene over that of di-σ-bonded ethylene on Pt(111) along with the decrease in rehybridization of chemisorbed ethylene on Pt/Sn alloys back toward that in the gas phase, and the slightly weaker Pt-H bond on Pt/Sn alloys,22 which should create a more-labile H adatom. To probe experimentally the influence of chemisorbed hydrogen on ethylene chemistry on Pt/Sn alloys, we have investigated the effect of hydrogen adatoms on adsorption, desorption, and reaction of ethylene on a (x3×x3)R30°Sn/Pt(111) surface alloy by using TPD and Auger electron spectroscopy (AES). We report these results herein. Because H2 does not adsorb on this alloy under UHV conditions,22 an H-atom source was used to generate the hydrogen-precoverages investigated. 2. Experimental Methods Experiments were performed in a three-level UHV chamber at a typical background pressure of 1 × 10-10 Torr, as described earlier.23 The Pt(111) crystal (Atomergic; 10 mm diam, 1.5 mm thick) was prepared by using 1-keV Ar+-ion sputtering and oxygen treatments (5 × 10-7 Torr O2, 900 K, 2 min) to give a clean spectrum using AES and a sharp (1 × 1) pattern in LEED. The (x3×x3)R30°Sn/Pt(111) surface alloy was prepared by evaporating two monolayers of Sn onto the Pt(111) crystal surface and subsequently annealing the sample to 830 K for 20 s. On this surface prepared as above, Sn is incorporated substitutionally into the surface layer to form an ordered alloy with θSn ) 0.33, corresponding to a Pt2Sn surface. This surface is relatively “flat”, but Sn atoms protrude 0.02 nm above the surface-Pt plane.24,25 Importantly, forming this alloy structure eliminates all pure-Pt, 3-fold sites. For brevity herein, we will refer to the (x3×x3)R30°-Sn/Pt(111) surface alloy as the x3 alloy. A Pt-tube doser was constructed, based on the design of Engel and Rieder,26 as a pyrolytic source of gas-phase hydrogen atoms. The principal component is a bent Pt tube (1 mm o.d., 0.8 mm i.d.) into which a hole of 0.1 mm diameter was mechanically drilled. The tube was resistively heated to 1275 °C while watercooling kept an adjacent Cu block cold. The temperature of the Pt tube was directly measured by an optical pyrometer that was calibrated by the temperature of the Pt(111) crystal sample, as measured by a Cr/Al thermocouple. The estimated relative accuracy of the pyrometer reading was (5 °C. The flux of H atoms obtained from this source with the Pt tube at 800 °C, and the H2 pressure increased in the Pt tube until a background pressure of 5 × 10-8 Torr was measured in the UHV chamber was estimated to be 3 × 1013 atoms cm-2 s-1. This value was obtained by assuming that the initial sticking coefficient of H atoms on Pt(111) at 100 K was unity and using the H adatom coverage and subsequent H2 yield in TPD produced from the well-known decomposition of ethylene on Pt(111).10 H2 (Matheson; 99.99%) was introduced via a variable leak valve (Granville-Phillips) into the Pt-tube doser after passing through a liquid-nitrogen-cooled, U-tube trap. C2H4 (Matheson; 99.99%) was introduced via a microcapillary array doser connected to the gas line through a leak valve. All of the exposures reported here are given simply in terms of the pressure measured by the ion gauge in the UHV chamber. No attempt was made to correct for the flux enhancement of the doser or ion gage sensitivity. Mass spectrometry performed in the UHV chamber showed no appreciable concentration of impurities in the source gases. For all TPD experiments, the heating rate was 3.6 K s-1 and all exposures were given with the surface temperature at 100 K. (22) Voss, M. R.; Busse, H.; Koel, B. E. Surf. Sci. 1998, 414, 330. (23) Zhao, H.; Kim, J.; Koel, B. E. Surf. Sci. 2003, 538, 147. (24) Overbury, S. H.; Mullins, D. R.; Paffett, M. T.; Koel, B. E. Surf. Sci. 1991, 254, 45. (25) Atrei, A.; Bardi, U.; Rovida, G.; Torrini, M.; Zanazzi, E.; Ross, P. N. Phys. Rev. B 1992, 46, 1649. (26) Engel, T.; Rieder, K. H. In Structural Studies of Surfaces; Ho¨hler, G., Ed.; Springer: Berlin, 1982; Vol. 91, p 55.

Figure 1. H2 (2 amu), ethylene (28 amu), and ethane (30 amu) TPD spectra following 0.24-L ethylene exposures on a (x3×x3)R30° Sn/Pt(111) surface alloy with 0.14-ML preadsorbed H at 100 K. AES measurements were made with a double-pass cylindrical mirror analyzer (CMA). The electron gun was operated at 3-keV beam energy and 1.5-µA beam current. Coverages, θi, reported herein are referenced to the surface atom density of Pt(111) such that θPt )1.0 ML is defined as 1.505 × 1015cm-2.

3. Results TPD was used to probe monolayer (saturation) coverages of ethylene on a clean and H-precovered x3 alloy at 100 K. Possible gas-phase reaction products formed during TPD were monitored at 2, 26, 28, 30 amu, but only the desorption of H2 (2 amu) and ethylene (28 amu) were identified, as shown in Figure 1. The most significant finding of this investigation is that the reaction pathway that is so prominent on Pt(111)3,13 between coadsorbed H adatoms and ethylene resulting in the evolution of gasphase ethane upon heating is totally blocked on the x3 alloy. Desorption spectra for the H2 and C2H4 products are shown in Figures 2 and 3. The desorption spectra of H2 were not perturbed to any significant extent by the subsequent coadsorption of 0.24-L ethylene, as shown in Figure 2. This ethylene exposure exceeds by at least 30% the amount required to saturate a “clean” x3 alloy surface, i.e., with θH ) 0, at 100 K. H2 desorption at low θH occurs at 265 K in a peak that shifts to 253 K with increasing θH. Upon saturation of this peak, a second peak at 215 K evolves at higher θH. The features below 200 K are in part from the cracking of ethylene in the mass spectrometer. The H2 desorption peaks are about 15 K lower than D2 desorption peaks reported previously on the x3 alloy,22 which is attributed to kinetic isotope effects (in addition

Influence of Coadsorbed Hydrogen on Ethylene Adsorption

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Figure 4. Influence of preadsorbed H adatoms on the monolayer (saturation) coverage of ethylene on a (x3×x3)R30° Sn/ Pt(111) surface alloy at 100 K.

Figure 2. H2 TPD spectra following 0.24-L ethylene exposures on a (x3×x3)R30° Sn/Pt(111) surface alloy at 100 K that has been precovered with varying amounts of H adatoms.

Figure 3. Ethylene TPD spectra following 0.24-L ethylene exposures on a (x3×x3)R30° Sn/Pt(111) surface alloy at 100 K that has been precovered with varying amounts of H adatoms.

to different heating rates). As shown in Figure 3 for small precoverages of H adatoms, subsequent ethylene exposures produced ethylene desorption in TPD in a peak at 170 K, which is 10 K lower than that from a clean x3 alloy. With increasing θH, ethylene desorption shifted to

lower temperatures, and eventually ethylene desorbed in a peak at 162 K for θH g 0.19 ML. An important observation shown in Figure 3 is that the intensity of the ethylene desorption peak decreases with increasing θH, and we will expand on this observation below. The small desorption feature observed between 350 and 400 K in Figure 3 is from CO desorption from adsorbed CO (θCO < 0.02 ML) accumulated from the background gas. Assuming firstorder kinetics with a preexponential factor of 1013 s-1, the Redhead method27 can be used to estimate a desorption activation energy Ed for ethylene. A value of Ed ) 45.5 kJ mol-1 was found for C2H4 adsorbed on the clean x3 alloy, and this was decreased to 40.8 kJ mol-1 on the larger H precoverages on the x3 alloy. In cases such as this where no appreciable barrier to adsorption is expected to exist, the desorption activation energy is equal to the adsorption energy and one sees that hydrogen weakens slightly (∼10%) the bonding interaction of coadsorbed ethylene to the alloy surface. Figure 4 summarizes the influence of preadsorbed H adatoms on the ethylene monolayer (saturation) coverage on the x3 alloy, as obtained by integration of the ethylene TPD peak areas. The saturation coverage of ethylene on the clean x3 alloy was set to 1.0 on the left axis, and this corresponds to an actual coverage of 0.27 ML.21 The actual ethylene coverages are provided on the right-hand axis. Ethylene coverage was decreased linearly with increasing amounts of preadsorbed H adatoms. For θH ) 0.30 ML, the ethylene saturation coverage is reduced to only 18% of that on the clean x3 alloy. Extrapolation of the line given in Figure 4 indicates that the chemisorption of diσ-bonded ethylene will be completely blocked when 0.35 ML of H adatoms is preadsorbed on the x3 alloy surface. To understand the quantitative aspects of the decrease in the saturation coverage of ethylene with increasing coverages of preadsorbed H on the x3 alloy, we also carried out one coadsorption experiment on Pt(111). AES was used to measure the amount of adsorbed ethylene on the surface with and without preadsorbed hydrogen. Figure 5 shows two AES spectra, one obtained after a saturation exposure of ethylene on clean Pt(111) (bottom) and the other after the same ethylene exposure on a Pt(111) surface that had been precovered by 0.6-ML H (top). The C(272):Pt(237) ratio shown for the bottom spectrum corresponds to θC ) 0.50 ML (θC2H4 ) 0.25 ML10) on clean Pt(111). This ratio (27) Redhead, P. A. Vacuum 1962, 12, 203.

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Figure 5. AES spectra obtained following adsorption of a monolayer (saturation) coverage of ethylene on clean Pt(111) (bottom) and a 0.6-ML H precovered Pt(111) surface (top).

decreases dramatically from that on Pt(111) to that of C(272):Pt(237) ) 0.17 for ethylene adsorbed on 0.6-ML H precovered Pt(111). This ratio corresponds to θC ) 0.15 ML (θC2H4 ) 0.07 ML). These data suggest that H adatoms effectively block ethylene access to Pt sites on both Pt(111) and the x3 alloy. Such site blocking effects of preadsorbed H have also been observed for benzene28 and 1,3-butadiene29 chemisorption on Pt(111). Preadsorbed hydrogen on Pt(111) also blocks ethylene chemisorption sites, but more hydrogen than on the alloy is required to eliminate chemisorbed ethylene on Pt(111) at 100 K. On the x3 alloy, preadsorbed θH ) 0.30 ML blocked 82% of ethylene adsorption sites while twice as much hydrogen, θH ) 0.60 ML, only blocked 70% of the ethylene adsorption sites on Pt(111). This means that ethylene cannot chemisorb on Sn sites or on many of the remaining Pt sites on the x3 alloy. On the x3 alloy, Sn only occupies 33% of the surface area and 67% is still available as Pt sites. Our results show that only 0.35 ML of preadsorbed H, which occupies one-half the available Pt sites, will totally block the adsorption of ethylene. This indicates that two Pt atoms are required for ethylene adsorption. 4. Discussion Hydrogen coadsorption weakens the interaction between ethylene and the (x3×x3)R30°-Sn/Pt(111) alloy surface. One effect is to cause a decrease in the adsorption energy of coadsorbed di-σ-bonded ethylene, but this perturbation is rather small. The most significant, and unexpected, effect of hydrogen coadsorption is to cause a nearly 1:1 “site-blocking” of ethylene chemisorption. Preadsorption of 0.3-ML H is sufficient to eliminate population of di-σ-bonded ethylene on the x3 alloy surface. There is a direct, large repulsive interaction between coadsorbed H adatoms and ethylene molecules on the x3 alloy.29 Such a geometric or site-blocking effect has often been invoked to explain the action of a second metal atom, such as Sn or Bi, or other adatoms, such as S, O, or C, on transition metal chemistry, but such an effect is rarely used in explaining the action of the relatively “small” H adatoms. The site-blocking behavior of H on the x3 alloy and Pt(111) is similar, although one might expect small differences to arise from the weaker, and thus longer, Pt-H bond on the alloy which allows the H adatom to sit higher at the surface or perhaps have a little additional (28) Lutterloh, C.; Biener, J.; Po¨hlmann, K.; Schenk, A.; Ku¨ppers, J. Surf. Sci. 1996, 352, 133. (29) Zhao, H.; Koel, B. E. Surf. Sci., submitted for publication.

Zhao and Koel

negative charge, and thus larger diameter, interfering with the close approach of ethylene to Pt atoms at the surface needed to facilitate strong bonding interactions. The large difference in the amount of H needed on the x3 alloy and Pt(111) to completely eliminate sites for ethylene to chemisorb arises from the influence of alloyed Sn. These surface Sn atoms directly eliminate Pt sites by their physical presence and also affect the electronic structure of neighboring Pt sites.30,31 Another possible influence is that preadsorbed H exerts an electronic or ligand effect in which H adatoms change the local density of states at Pt sites on the x3 alloy, and this inhibits back-bonding so severely that di-σ-bonded ethylene cannot be formed. There is support for this notion. RAIRS has been used to show that di-σ-bonded ethylene on Pd(111) is converted to π-bonded ethylene when the surface is presaturated by H adatoms.32 First-principles calculation of ethylene and H coadsorption on Pd(110)33 shows that the presence of coadsorbed H increases the four-electron repulsion between the ethylene π orbital and the dz2 band of the metal and decreases the back-donation interaction between the metal dz2 orbital and the ethylene π* orbital. Because ethylene is already less rehybridized on the x3 alloy than on Pt(111), the addition of coadsorbed hydrogen could reduce the back-bonding just enough to convert di-σ-bonded ethylene to π-bonded ethylene, which is only physisorbed on the x3 alloy and therefore not adsorbed at 100 K (desorbing at 85 K on K/Pt(111)34). The other major finding of this work is the large difference in hydrogenation activity in these experiments between the Pt(111) surface and the x3 alloy. Coadsorption of H adatoms and ethylene on Pt(111) forms mostly ethane upon heating.3,13 This reaction pathway is totally blocked on the x3 alloy. This is unexpected. Alloyed Sn in Pt(111) shifts the sp3-hybridization of carbon atoms in di-σ-bonded ethylene in the direction of sp2-hybridized carbon in π-bonded ethylene,20 and it has been reported that π-bonded ethylene hydrogenates to ethane much faster than di-σ-bonded ethylene under catalytic conditions on Pt(111).8 However, Domen’s recent IRAS findings35 excluded the existence of π-bonded ethylene under similar catalytic conditions, and they concluded that the ratedetermining step of hydrogenation does not involve forming either di-σ-bonded or π-bonded ethylene (they proposed that activation of hydrogen plays an important role in ethylene hydrogenation under catalytic conditions). Our finding that no ethylene hydrogenation occurs upon heating a coadsorbed monolayer of H adatoms and ethylene on the x3 alloy implies that π-bonded ethylene is not particularly reactive and ethylene simply desorbs at a temperature that is lower than that required for hydrogenation. Our results have important implications for modeling and understanding catalytic hydrogenation over bimetallic Pt/Sn catalysts. Adding Sn to supported Pt catalysts causes a dramatic decrease in catalytic activity for the propene (C3H6) hydrogenation reaction.36 The hydrogenation activity for ethylene and 1-hexene (C6H12) is also inhibited (30) Pick, S. Surf. Sci. 1999, 436, 220. (31) Batzill, M.; Beck, D. E.; Koel, B. E. Surf. Sci. 2000, 466, L821. (32) Stacchiola, D.; Azad, S.; Burkholder, L.; Tysoe, W. T. J. Phys. Chem. B 2001, 105, 11233. (33) Fihol, J. S.; Simon, D.; Sautet, P. J. Phys. Chem. B 2003, 107, 1604. (34) Cassuto, A.; Touffaire, M.; Hugenschmidt, M.; Dolle, P.; Jupille, J. Vacuum 1990, 41, 161. (35) Ohtani, T.; Kubota, J.; Kondo, J. N.; Hirose, C.; Domen, K. J. Phys. Chem. B 1999, 103, 4562. (36) Galvagno, M.; Staiti, P.; Antonucci, P.; Donato, A.; Pietropaolo, R. J. Chem. Soc, Faraday Trans. 1983, 79, 2605.

Influence of Coadsorbed Hydrogen on Ethylene Adsorption

on Pt/Sn/Al2O3 catalysts.37 Our surface science results are consistent with these observations, even though Pt/Sn alloy phases have not been identified as catalytically active in these catalysts. There has been one report in a model study that the addition of 0.1-ML Sn increased the reactivity of Pt(111) for ethylene hydrogenation.18 However, these authors themselves pointed out the difficulty of distinguishing a direct effect from changes that occurred in the amount of carbon deposition, and reaction rate decreased quickly with further increases of Sn. We conclude that adding Sn to platinum catalysts decreases the alkene hydrogenation activity by not only decreasing the interaction between alkenes and Pt but also, more importantly, by decreasing the reactivity of adsorbed alkenes at the catalyst surface.

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completely inhibited on a (x3×x3)R30°-Sn/Pt(111) surface alloy with θSn ) 0.33. Coadsorbed H on the alloy slightly weakens (∼10%) the adsorption energy for ethylene on the this alloy. However, the chemisorbed monolayer saturation coverage of ethylene decreased linearly with increasing θH on the alloy due to substantial siteblocking effects, which totally eliminate ethylene chemisorption at θH ) 0.35 ML. This is much less hydrogen than is needed to completely block the adsorption of ethylene on Pt(111). These observations represent important information for advancing the surface science of hydrogenation and dehydrogenation catalysis and aid in explaining the decreased activity for alkene hydrogenation observed in supported bimetallic Pt/Sn catalysts.

5. Conclusions The hydrogenation reaction pathway so prevalent on Pt(111) between ethylene and coadsorbed hydrogen is (37) Palazov, A.; Bonev, Ch.; Shopov, D.; Lietz, G.; Sa´rka´ny, A.; Vo¨lter, J. J. Catal. 1987, 103, 249.

Acknowledgment. This work was supported by the Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division. LA048035Q